SRTM Global Digital Elevation Model (South West, one of six data sets) represents an elevation map (between 60 degrees North and 56 degrees South latitude) of the world from the NASA/NGA Shuttle Radar Topography Mission (SRTM) data sets from the U.S. Geological Survey's EROS Data Center. The resolution is 3 arc seconds (90 meters). The pixel value represents the elevation in meters.
SRTM Global Digital Elevation Model provides a base map layer displaying elevation information between 60 degrees North and 56 degrees South latitude for geographic analysis on regional and national scales.
Largest scale when displaying the data: 1:250,000.
ground condition: 200002; publication date: 20050401
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After processing, the data set is checked for drawing display and file sizes compared with source materials.
Conforms to the horizontal positional accuracy of the Shuttle Radar Topography Mission DTEDr Level 1 (3-arc second) Data.
The SRTM vertical datum is mean sea level and is based on the WGS84 Earth Gravitational Model (EGM 96) geoid. The elevation values of the SRTM data (excluding void filled areas) are based on first reflective return, which means that the elevation value may represent bare earth, vegetation, or man-made features. The absolute height accuracy of the source SRTM data (excluding void filled areas) and prior to compression is "better than" 16 meters (at a 90% confidence level). In order to distribute the elevation data on a single (dual layer) DVD9 it was necessary to compress the SRTM data using lossy JPEG2000 compression. The compression quality setting (5) introduced a vertical bias to the data. Based on a test of the Western United States against the original data values this equates to a RMS error of 2.15 (excluding void filled areas). The minimum difference was 0 meters and the maximum difference was +/- 30 meters.
Attribute and geospatial data
Attribute and geospatial data
SRTM DTED is a uniform matrix of elevation values indexed to specific points on the ground. The horizontal datum is the World Geodetic System 1984 (WGS84) and the vertical datum is mean sea level as determined by the WGS84 Earth Gravitational Model (EGM 96) geoid. The elevation data are with respect to the reflective surface, which may be vegetation, man-made features or bare earth. DTED Level 2 elevation values are spaced one arc second apart between 0 degrees and 50 degrees latitude, and spaced one arc second apart in latitude and two arc seconds apart in longitude between 50 degrees and 60 degrees latitude. SRTM DTED Level 1 values are derived from the SRTM DTEDr 2 values such that the DTED 1 values are identical to the DTED 2 values at coincident points. However, the SRTM DTED 1 values are spaced 3 arc seconds apart between 0 degrees and 50 degrees latitude, and spaced 3 arc seconds apart in latitude and 6 arc seconds apart in longitude between 50 degrees and 60 degrees latitude. After NASA/JPL completes the raw data processing, NGA performs quality assurance checks on the JPL SRTM data and then carries out several additional finishing steps. Spikes and wells in the data are detected and voided out if they exceed 100 meters compared to surrounding elevations. Small voids (16 contiguous posts or less) are filled by interpolation of surrounding elevations. Large voids are left in the data. Water bodies are depicted in the SRTM DTED. The ocean elevation is set to 0 meters. Lakes of 600 meters or more in length are flattened and set to a constant height. Rivers that exceed 183 meters in width are delineated and monotonically stepped down in height. Islands are depicted if they have a major axis exceeding 300 meters or the relief is greater than 15 meters. The data are processed in one degree by one degree "cells". The edges of cell are matched with the edges of adjacent cells to assure continuity. In addition to voids due to shadows and layover and poor reflective properties of the Earth's surface, there are occasional floating islands of data, unregistered vertically, due to phase unwrapping errors. On the whole, the SRTM DTED absolute height accuracy is significantly better than the 16 meters (90 % confidence) specification for the mission and the data are 95% complete over the collection area.
The following steps were performed by ESRI: Ordered and received the Shuttle Radar Topography Mission DTEDr Level 1 (3-arc second) elevation data on thirteen DVDs (14,277 DTEDr cells) from the U.S. Geological Survey Center for Earth Resource Observation and Science (EROS). Converted each one degree by one degree elevation DTED cell from DTED format to ESRI Grid format. Created a NoData ESRI Grid from each elevation ESRI Grid; this was used to identify which elevation ESRI Grids required void fill-ins. Within certain elevation ESRI Grids that contained voids, some voids existed in the water/land "interface" (i.e. a void that spatially spanned across water and land). The assumed water areas within this type of void were manually identified and the elevation ESRI Grid pixels were set to a constant elevation value that was appropriate for the area. Filled all voids within the one degree by one degree elevation ESRI Grids. The primary method used to fill voids was the Delta Surface Fill (DSF) method (see the article "Filling SRTM Voids: The Delta Surface Fill Method" by Greg Grohman, George Kroenung and John Strebeck in the March 2006 issue of "Photogrammetric Engineering & Remote Sensing"). The GTOPO30 elevation dataset was used as the fill-in source. How the DSF method differs from other interpolation methods is that with the DSF method the voids are not filled by interpolating through the voids in the elevation, but rather interpolating through the voids within a delta surface, which is the difference between the surface requiring fill (SRTM) and the fill-in source (GTOPO30). The interpolated surface is used to adjust the GTOPO30 fill-in source, which values are then used to fill-in the voids within the SRTM elevation surface. In situations where there were no GTOPO30 pixels in the area of a void, the DSF method was unable to fill these voids. In these cases a secondary method was used to the fill the voids. This involved interpolating through the voids in the elevation values in the output surface from the DSF method. Mosaicked all SRTM elevation ESRI Grids (void-filled and those that did not require fill-in) into a single ArcSDE 9.1 raster dataset. NOTE: There were six ESRI Grids where the extent exceeded the -180ø longitude; these ESRI Grids were clipped at the -180ø longitude prior to mosaicking. The process of mosaicking created NoData areas in the elevation raster dataset in the area of two inland water bodies (elevation not 0 meters). Created and loaded constant elevation ESRI Grids into the ArcSDE elevation raster dataset for the following areas (constant elevation noted): Lake Superior (179 meters) and Caspian Sea (-29 meters). The SRTM data in DTED Level 1 format did not include data for the cell E012N24 (South West Libya). In order to fill this area with elevation data, the GTOPO30 dataset was processed in the following manner: Resampled to match SRTM elevation ESRI Grid cellsize; Smoothed using a 9 by 9 rectangular focalmean function; created a delta ESRI Grid by subtracting the resampled and smoothed GTOPO30 ESRI Grid from an ESRI Grid containing the five rows and columns worth of SRTM data surrounding the E012N24 cell; adjusted the resampled and smoothed GTOPO30 ESRI Grid based on the statistical mean of the delta ESRI Grid; clipped the adjusted GTOPO30 ESRI Grid to match the missing extent of elevation data; converted the adjusted GTOPO30 ESRI Grid from floating point to integer; and loaded the resultant ESRI Grid into the ArcSDE 9.1 elevation raster dataset. Calculated raster statistics on the ArcSDE 9.1 elevation raster dataset. Extracted data from the ArcSDE 9.1 elevation raster dataset into the six ESRI Grids that represent the final delivery extents. For each of these elevation ESRI Grids (except for the South East ESRI Grid), set the NoData pixels to have a value of 0 (these pixels are in the areas formed by the mosaicking of the SRTM elevation ESRI Grids and are located in the ocean). The South East ESRI Grid contained islands that were located along the 180ø longitude, but because of the differing cell alignment above/below 50 degrees north/south latitude within the original DTED data, the mosaicked data had the last column populated with NoData. For this South East ESRI Grid set all NoData pixels to 0 except for the last column; for the last column used the focalmean function to add back land values. Converted the six elevation ESRI Grids to 16-bit signed integer JPEG 2000 raster datasets using a compression quality of 5. Calculated raster statistics for the elevation JPEG 2000 raster datasets. Created ArcGISr layer files (.lyr).
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See use constraints.
The JP2 file contains the image data. The AUX file contains auxiliary information (optional) (i.e. information from ArcGIS, which the JP2 format cannot store natively). The XML file (*.jp2.xml) contains the metadata describing the data set (optional).
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